The United States is the world's largest producer of corn. US production reached 9.5 billion bushels in 2001, greatly exceeding the production of any other grain (National Corn Growers Association 2002). Direct use as animal feed is the largest consuming application, accounting for 5.85 billion bushels in 2001. Corn processing, either via wet milling into sweeteners, starch, ethanol and other industrial products, or via dry milling for ethanol production, accounted for 1.7 billion bushels of US consumption in 2001, or slightly less than 18% of the crop.
Corn processing is expected to increase significantly over the next decade. Ethanol production has been the largest single application of corn processing since 1999, reflecting the recent high growth of the fuel ethanol market and the slowing growth of sweetener markets. Legislative and lobbying efforts are promoting a renewable fuels standard for gasoline. Most projections are for a three fold increase in ethanol production, accounting for another 1.4 billion bushels of corn consumption if no significant changes are made to existing manufacturing processes. Other new industrial uses for corn, such as biodegradable plastics, will also contribute to the expansion of the corn processing industry.
A conventional corn dry milling process with the production of ethanol is illustrated in FIG. 1. Briefly, the process involves an initial preparation step of grinding and cooking the corn 110. The resulting product is subjected to a step of hydrolysis 120. Then a yeast fermentation is conducted for the production of ethanol 130. The ethanol is distilled from the fermentation broth 140 and dried 150. The remainder of the fermentation medium is then dried to produce DDGS 160. This step typically includes a solid/liquid separation 170, wherein the liquid stream is subjected to an evaporation step 180 to concentrate soluble byproducts, such as sugars, glycerol and amino acids, before being recombined with the solids to be dried into DDGS.
The economics of corn processing are dependent to a great degree on the value of the animal feed coproducts made from the non-starch fractions of corn. The projected expansion of the industry will likely result in severe oversupply conditions in the coproduct markets unless changes are made. In particular with the expansion of the dry mill ethanol industry there will be a huge increase in supply of distiller's dried grains and solubles (DDGS). There are several limitations on the utilization of DDGS, including limited export markets which are hampered by continuing disputes with trading partners, and the limited market range in terms of animal feeding systems, essentially limited to ruminants.
It would be desirable to increase the value of the coproducts and to expand the range of markets beyond ruminant animals. One way to do this is to utilize the fiber fraction of the corn. Various schemes have been derived to hydrolyze the fiber fraction of the corn for utilization as a fermentation substrate. However the fiber fraction is complex and is composed of hemicellulose and cellulose components and yields a hydrolysate, upon enzymatic hydrolysis, for example, that is of limited use as a substrate in the conventional yeast based ethanol process. Thus it would be desirable to have a process that could utilize the complex mixture of components from corn fiber hydrolysate. It would also be desirable to utilize the metabolic byproducts from the yeast fermentation such as glycerol which is a difficult material to handle in the ethanol plant particularly in drying of DDGS. If the fiber fraction of the corn could be utilized it would increase the yield from the plant and concentrate and thus increase the value of the corn protein fraction of the corn. The increased concentration of protein with lower fiber content is a higher value animal feed coproduct. For example, corn gluten meal is a fraction of corn protein produced in corn wet milling that has low fiber content, a higher protein concentration than DDGS, contains the pigments from the corn, and has a market value of greater than twice DDGS. Corn gluten meal can also be utilized in a broader range of markets than DDGS such as chickens and other monogastric animals. By selectively removing the fiber fraction from DDGS, a product more like corn gluten meal would be produced with higher value.
In addition, since there are a large number of already existing ethanol plants based on corn dry milling, and the number is increasing rapidly, it would be desirable to have a process that could be integrated with these plants to improve the product value, increase yield and utilize to the greatest extent possible the existing physical plant assets.
Thus, a need exists for improvements in the corn dry milling process to increase ethanol production and to develop byproducts of higher value and with broader markets than DDGS.